Invar Tape

In this photograph, surveyors use an invar tape to measure a base line through a house in Pasadena, California. Invar, an alloy of nickel and steel having a very low coefficient of expansion, resolved the thermal expansion problem associated with the use of other metal rods and tapes for distance measurement. With the invention of invar, tapes and wires became feasible for measuring distances. The Coast and Geodetic Survey began measuring distances using invar tapes in the early 1900s. These tapes would eventually replace steel tapes as the tool for taking high accuracy measurements.

Where there's a will, there's a way! And,
hopefully, an understanding home owner. Here surveyors use an invar
tape to take a measurement through a house!

As with metal bars, temperature variation was by far the biggest problem
with using tapes for distance measurements. In 1905, the Coast
and Geodetic Survey (C&GS) purchased tapes made of a nickel-steel alloy
invented in 1896 by French physicist Charles Édouard Guillaume. The
alloy was named invar, from invariable, to reflect its
low coefficient of expansion.

Invar had several advantages over steel tapes. It took a thermometer
error of 2.8° C to create the same error in invar as a 0.1° C error
would cause in a steel tape. Invar could be used during
the day, which meant that it was easier to read and to keep the tapes clear
of obstructions and moisture. Working and traveling during the day
made for much faster progress than the nighttime measurements required by
steel. Although the softer metal could kink near the ends if the handler
was not careful, invar tapes were just as accurate and could be handled
and manipulated in the same way as steel tapes.

Compared with previous tools, making measurements with invar
cost half as much. After comparisons on six different base lines in
1906, invar completely replaced steel for high accuracy distance measuring
in the C&GS.

Historic Reference

The 22-mile long Pasadena base line
being measured in the photograph was done to assist in studies
of the speed of light, in the hope that the experiments would lead
to a way to use light to measure distances. At the time, this base
line was considered the most accurately determined distance
in any survey in the world.

Distance Measurement Instrument Shown: Invar tape

Location: Pasadena, California

Manufacture Date: Early 1900s

Dates of Use: Early 1900s - Present

Photo Date: 1922

Works Consulted

French, O. B. (1907). Six Primary Bases Measured with Steel and Invar
Tapes [Electronic version]. Report of the Superintendent of the
Coast and Geodetic Survey Showing the Progress of the Work From July 1,
1906, to June 30, 1907. p 105-155.